Heretofore, glass has been widely used as a material for forming containers for seasonings, oils, juices, carbonated drinks, beer, Japanese sake, cosmetics, detergents, etc. Usually, however, because of relatively high production costs of glass containers, empty containers after use are recovered and circulated for re-use. Furthermore, glass containers had such drawbacks that because they are heavy, costs of transport increase and, in addition thereto, they are easy to break and difficult to handle.
In order to solve such drawbacks associated with glass containers, various plastic containers have recently come to be used rapidly in substitution for glass containers. Various plastics are used as materials for plastic containers according to the kind of the contents to be contained and to the purpose for which the contents are used. Of such plastics materials, saturated crystalline polyester resins such as polyethylene terephthalate and the like are excellent in mechanical strength, heat resistance, transparency and gas barrier properties, and are used as materials for containers for juices, cooling beverages, carbonated drinks, seasonings, detergents, cosmetics and the like. Of the plastic containers used for such purposes as mentioned above, blow molded containers for juices, cooling beverages and carbonated drinks are required to be subjected to sterilizing and high-temperature filling. On that account, the blow molded containers must be molded from heat resistant resins capable of withstanding the high-temperature filling operation, and such blow molded containers for filling purposes are required to be excellent in transparency and form stability such as uniformity in internal space.
As stated above, saturated crystalline polyester resins are plastics excellent in physical properties, but no saturated crystalline polyester resins simultaneously having the above-mentioned transparency, heat resistance capable of withstanding the high-temperature filling operation and form stability have been known heretofore.
Especially, there was such a serious problem that when a saturated crystalline polyester resin such as polyethylene terephthalate is fed to a molding machine such as an injection molding machine to prepare a preform for blow molded article, and this preform is then blow molded into a blow molded container, the resulting blow molded container chalks and deteriorates in transparency. On that account, the blow molded containers that were deteriorated in transparency are unavoidably discarded, resulting in a sharp decrease of the yield of the products.
There have been proposed various processes for molding saturated crystalline polyester resins such as polyethylene terephthalate into blow molded containers which are heat resistant, for example, a process in which heat resistant resins such as polyallylate are laminated on the polyester blow molded containers (Plastics, Vol. 1, 36 (No. 9), 121 (1985), a process in which the polyester blow molded containers as molded are subjected to heat setting (Japanese Patent Publn. No. 3301/1984, Japanese Patent L-O-P Publns. Nos. 12031/1980, 75833/1981 and 13142/1981) and a process in which the containers as molded are improved in crystallinity by subjecting them to solvent treatment. These processes as cited above are all intended to impart heat resistance to saturated crystalline polyester resins such as polyethylene terephthalate which are inherently insufficient in heat resistance by means of molding means or treatment after molding. In all the blow molded containers obtained by every one of the above-cited processes, the their transparency and form stability are not sufficiently satisfactory, and hence the advent of blow molded containers consisting essentially of saturated crystalline polyester resins and having excellent transparency and form stability has been earnestly desired.
With the view of obtaining blow molded containers excellent in transparency and form stability from saturated polyester resins, the present inventors prosecuted extensive research. As a result, they have found that the reason why blow molded containers consisting of saturated crystalline polyester resins chalk and decrease in transparency and further decrease in form stability is that in a process of preparing a preform for blow molded article from saturated crystalline polyester resins by injection molding or the like, said saturated crystalline polyester resins are heated, fused and sheared, whereby the crystallizing temperature (Tc) of the saturated crystalline polyester resins decreases, and that blow molded articles obtained from the preform consisting of saturated crystalline polyester resins having a decreased crystallizing temperature (Tc) will come to chalk. On the basis of the technical information obtained above, the present inventors continued a further investigation and have eventually accomplished the present invention based on their finding that if saturated crystalline polyester resins are molded into molded articles using a molding machine under specific conditions, the crystallizing temperature of said unsaturated crystalline polyester resins will not decrease, whereby the molded articles obtained, such as blow molded articles, are found to be excellent in transparence and form stability.